Equipment Description:Drainage Aids
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Drainage aids are primarily used to promote the removal of water from the paper stock on the forming wire. Water removal is primarily brought about by gravity through the wire, and secondarily by the mechanical action of elements such table rolls, foils and vacuum boxes. The intention is to create a wet-web with good formation prior to entry into the press section. To achieve good formation, the initial drainage of the wet-web should not be too rapid. It is important to carry the benefits of drainage on the wire through into the press section. There is no point having excellent drainage on the wire if water removal is then impeded through the presses. Improvement in water release from the paper stock allows machines to run faster, while at the same time the amount of energy required in the vacuum and drying sections is significantly reduced. |
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Note that an efficient retention aid system should also promote stock drainage. However, it is possible to increase retention without improving overall water removal through the wire and press sections. Drainage aids can be either single shot or multi-component products, usually chosen to also improve first pass and ash retentions. The general description relating to Retention Aids also applies to Drainage Aids. Drainage aids are applied in the stock approach flow of the paper machine. As with any chemical, it is important not to over-dose the drainage aid. 1. Too much drainage aid means higher costs. 2. If drainage on the wire is too fast, the formation of the sheet can be fixed before the web reaches the mechanical elements on the wire. This results in poor sheet formation. 3. Too much drainage aid can cause over-flocculation of the stock. This results in poor sheet formation. 4. Too much drainage aid can result in free polymer in the aqueous phase. This impedes water removal, both through the wire and in the press section. 5. Over-flocculating the stock can result in water being trapped within the sheet voids. This makes water removal at the press section difficult. It has been found that drainage is not a major issue on modern, high speed, twin wire machines. This has resulted in the interdependence of drainage aids and retention aids being reconsidered. Solid grade products are typically supplied in 25kg bags, 1 tonne big bags, and bulk deliveries into silos. Liquid grade solutions, emulsions and liquid dispersions can be obtained in 200kg plastic drums, 1 tonne reinforced semi-bulk containers or bulk delivery for transfer to silos. Emulsions require stirring to prevent settling. Liquid products may need protection from frost. Good practice should be followed when handling drainage aids, as well as adhering to any special instructions on the suppliers health and safety data sheet. |
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